The world turned a skeptical eye toward Syria on Thursday after a truce cast relative calm over restive cities and towns previously pounded by government forces.

"Syria is apparently experiencing a rare moment of calm on the ground," said Kofi Annan, the special envoy who brokered Syria's peace plan.

"This is bringing much-needed relief and hope to the Syrian people who have suffered so much for so long in this brutal conflict. This must now be sustained."

However, Susan Rice, the U.S. ambassador to the United Nations, said Annan told U.N. Security Council members that Syria lacks full compliance with the peace plan and that troops and heavy weapons remain in population centers despite an agreement to withdraw.

As talk of the need for international observers increased, U.S. Secretary of State Hillary Clinton said Syrian President Bashar al-Assad did not have the luxury to pick and choose what to implement in the peace plan.

"The Annan plan is not a menu of options," she said. "It is a set of obligations."

White House press secretary Jay Carney called the truce tentative.

"We cannot call what's happening on the ground a full cease-fire," he told reporters. "A tentative cease-fire, a less-than-full cease-fire, is not equal to a full implementation of the regime's obligations under the Annan plan."

The six-point Annan plan calls for the release of detainees, allowing access into the country for humanitarian aid and international media, and respecting the rights of peaceful demonstrators.

The Security Council on Friday may vote on a draft resolution that demands Syria to allow the deployment of up to 30 international observers and allow them unimpeded freedom of movement.

The draft also calls on all parties to cease armed violence.

Government guns fell mostly silent Thursday, but Syrian opposition groups still reported sporadic violence, and Thursday was not a day entirely free of bloodshed. At least 37 people were killed, said the Local Coordination Committees (LCC), a network of activists across Syria. Twenty-five of those deaths occurred in Homs, the group said.

The LCC reported the presence of government forces in one Damascus suburb and gunfire in two others. It reported mortar shelling in Madeeq Citadel in Homs.

The state-run Syrian Arab News Agency said an explosive device killed an officer and wounded 24 other people. It also said an "armed terrorist group" killed a Daraa citizen out buying bread.

Amid the tenuous truce, opposition groups called for peaceful demonstrations, seemingly testing whether the government would stick to its word.

Ban said he was working with the U.N. Security Council to send in observers as promptly as possible.

"The onus is on the government of Syria to prove that their words will be matched by their deeds this time," Ban said at a news conference in Geneva. "The world is watching, however, with skeptical eyes since many promises previously made by the government of Syria have not been kept."

French Foreign Minister Alain Juppe said an observer mission would "check whether the commitments made by the parties -- essentially the regime, because it is primarily responsible for the crackdown -- are actually being honored."

Russian Ambassador to the U.N. Vitaly Churkin said it was crucial for monitors to be on the ground.

Anti-regime protesters, meanwhile, came out onto the streets in several cities including Idlib, Homs, Deir Ezzor, Raqqa, Aleppo and Damascus, according to the LCC. It said troops opened fire at a checkpoint in Hama to prevent people from protesting.

Tanks were still roaming the streets, and snipers remained perched on rooftops in some cities, according to activists.

CNN is unable to independently verify reports of violence and deaths as the government has severely restricted access by international media.

The continued troop presence meant that al-Assad's guns could be pointed back at people within a moment's notice, and fears rose that large protests could spark violence.

Many Syrians and international observers questioned whether al-Assad would adhere to the cease-fire, especially after the government made it clear that it reserved the right to fire back on aggressors.

The regime has reneged on previous promises to stop violence, and al-Assad has described anti-government rebels as armed terror groups spreading mayhem in the country.

"Armed terrorist groups escalate their criminal operations in an attempt to damage Syria's stability and Annan's plan," said a headline on Syrian state-run television.

Bashar Jaafari, Syria's ambassador to the United Nations, told reporters Thursday that armed groups had eight violations of the cease-fire that resulted in several deaths Thursday.

Jaafari reiterated "again and again and again" his government's commitment to a peace plan and said Syrian troops had withdrawn from cities and towns.

"The credibility of the Syrian government has been confirmed," he said, rebutting Annan's statement that Syrian troops had not withdrawn.

"He didn't say that," Jaafari told reporters.

Rice, however, told reporters she couldn't imagine how the Syrian ambassador, who wasn't in the room when Annan was speaking, could characterize accurately what the former U.N. secretary-general said.

Jaafari also said the government is expecting armed opposition groups to end violence.

UK Foreign Minister William Hague said Thursday's lessening of violence "is a first fragile step toward peace that needs to be strengthened and sustained."

For the most part, after months of slaughter, residents of hotspots throughout the beleaguered nation woke up to the quietest day in months.

"Interestingly, we have been only hearing the sounds of birds," said Abu Salah, a Homs resident. "However, we have seen the security forces on alert."

Annan's plan, which appeared to falter in the days ahead of its Thursday deadline, is widely seen as the last chance for a diplomatic solution to the yearlong crisis in Syria.

Opposition fighters stayed in some cities to ward off attacks by government forces, activists said.

"The Free Syrian Army members remain in Homs, but there have been no clashes," activist Saif al-Arab said in reference to the army of military defectors. "They perceive their role as only to defend civilians in the face of any raids."

Just before the deadline, a tank fired mortar rounds into a residential area in Zabadani, 20 miles outside of Damascus, activists said.

"It was as if Assad's forces wanted to send a last message of warning," said Rania, a local activist who did not want to use a full name for safety reasons.

Opposition groups say deaths have occurred daily at the hands of al-Assad's forces. The day before the deadline, government forces attacked several cities, killing at least 98 people, the LCC reported.

Syria's anti-government protests erupted in March 2011, followed by a bloody government crackdown.

The United Nations estimates at least 9,000 people have died in Syria since the protests began. The LCC puts the death toll at more than 11,000.

Adib al Shishakly, a member of the Syrian National Council, an umbrella group of exiles, said Damascus needs to abide by all six points of Annan's peace plan.

"They diluted the whole initiative into one thing: into the cease-fire only," al Shishakly said. "What happened to the other five?"